Question

Delphi Threads with TBItmap, TJpegImage, TGraphics32

Asked by: Xor2

Hello,

   I have a procedure which performs some operation on jpg files (like transformations, etc).

I am;

  - reading pixel from canvas
       ACanvas.Pixels[X,Y];
  - drawing on canvas
       Bitmap.Canvas.Draw(0,0,jpg);
  - assign bitmap from another source
       Bitmap.Assign(LBitmapPomn);

The problem is that when i Run this procedure in a one thread everything is perfectly allright. I don't have memory leaks nor I have increased number of GDI objects and
the memory in the process manager varies from 50 to 100 mb and after this it returns to its default value.

The problem starts when i want to run this procedure in 4 threads. I have memory leaks. Memory inscreased to 1.7 gb, gdi +500 objects and so on..

I know only that Graphics32 component is thread-safe.
What about TBitmap, TJpegImage?
Which operation with this classes I should "Lock", should I lock?

Could the memory leak occur because of using these components in more than 1 thread?

How Can i fix my code?
Can I,  for instance use Graphics32 canvas without locking it.
Can I perform all this task only with Graphics32, which would gurantee thred safeness?

Thanks for help in advance..

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Asked On
2008-12-22 at 06:34:41ID24003128
Tags

Delphi

,

Thread

,

TBitmap

,

Graphics32

,

TBitmap32

Topics

Graphics and Delphi Programming

,

Delphi IDE

Participating Experts
2
Points
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Comments
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Answers

 

by: ciulyPosted on 2008-12-22 at 07:47:12ID: 23227119

TBitmap, TJpegImage are not trehadsafe. they are part of VCL, which is not thread safe :)
in order to use them in a trheaded environment, you must lock the resources, in most cases the canvas.

also, if you are looking for speed, you migt want to consdier using scanline instead of the pixels property (that is, if you are using many pixels from one line).

 

by: jgbustosPosted on 2008-12-22 at 07:52:43ID: 23227148

Hello,

A couple of years ago, I maintained an application that handled JPEG files using the TJPEGImage class in multiple worker threads. We used to have weird AVs and memory leaks, which we traced to the fact that the method TJPEGImage.Draw is notoriously thread-unsafe.

The code snippet is the contents of a file called ThreadSafeJpeg.pas, which solved the issues simply by creating a new class that inherits from TJPEGImage. The only method of the new class is an override to Draw, which wraps the parent method in calls to Lock and Unlock the canvas.

I'm not sure whether it will help, but it's worth a try.

unit ThreadSafeJpeg;
 
interface
 
uses
  Windows, Graphics, Jpeg;
 
type
  TThreadSafeJpegImage = class(TJPEGImage)
  protected
    procedure Draw(ACanvas: TCanvas; const Rect: TRect); override;
  end;
 
implementation
 
{ TThreadSafeJpegImage }
 
procedure TThreadSafeJpegImage.Draw(ACanvas: TCanvas; const Rect: TRect);
begin
  ACanvas.Lock;
  try
    inherited Draw(ACanvas, Rect);
  finally
    ACanvas.Unlock;
  end;
end;
 
end.

                                              
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by: Xor2Posted on 2009-01-13 at 04:01:48ID: 23361749

I wanted to add soemthing to this question.

The code which was proposed by jgbustos seems not to work, however very similar one works like a charm.
Memory is deallocated all the time and everything seems to work perfect.

procedure TThreadSafeJpegImage.Draw(ACanvas: TCanvas; const Rect: TRect);
begin
  Bitmap.Canvas.Lock; 
  try
    inherited Draw(ACanvas, Rect);
  finally
    Bitmap.Canvas.Unlock; 
  end;
end;

                                              
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